With widespread of portable audio players, in order to obtain satisfactory playback sound field space at the outdoors, noise reduction systems have recently been spread which can reduce noise of an external environment.
As such a noise reduction system, systems disclosed in the following Patent Literatures 1 to 3 are proposed, for example.
To be specific, the following Patent Literature 1 discloses a noise reduction system which reduces a noise component included in a frequency band having frequencies less than or equal to a specific value. Further, the following Patent Literature 2 discloses a noise reduction system which generates a noise reduction signal using noise collected by a microphone provided in an inner part of a housing worn by a user on his/her ear and noise collected by a microphone provided in an outer part of the housing.
Further, studies have been conducted on generating a more appropriate noise reduction signal in response to noise of an external environment. For example, the following Patent Literature 3 discloses that a noise reduction signal is generated by analyzing characteristics of collected noise and using a filter coefficient selected on the basis of the analyzed noise characteristics.